The present invention relates to dividing light into two or more beams so that the beams may be used for a different purpose such as forming an image and measuring the light.
It has already been known to perform light measurement by extracting a part of an information bearing image forming beam with a half-mirror or a perforated mirror. At times the surface of a transparent solid body has been provided with a small prism to retract a part of beam.
However it is inevitable that the image formed is darkened considerably by installation of a perforated mirror in the image forming beam. Further to install a mirror obliquely in the image forming beam requires much space. Such space is limited inside of an optical instrument.
On the other hand it is inevitable to produce a bad influence of the image forming ability to install a transparent solid body having a small prism in the image forming light flux.
Further in order that the light measurement might be carried out at the same time at several parts of the image forming beam or that the light measurement might be carried out at certain determined important parts of the beam, the manufacture of the light deviding means has been extremely difficult.